The invention concerns a rear-side film for solar modules, which are embedded in transparent EVA (ethylene vinyl acetate copolymer) that is crosslinked at a temperature of more than 140° with a rear-side film contacting the EVA and being bonded thereby, wherein the rear-side film consists in a multi-layer configuration with co-extruded layers of polyolefin that are directly connected with each other, of which a front layer with a layer thickness of less than 100 μm is filled with pigments or reflecting particles in a weight proportion of up to 20% and is reinforced with subsequent layers to a total thickness of the rear-side film of more than 300 μm. Solar modules represent plate-shaped units that can be mounted individually or in group arrangement in which electrically interconnected solar cells are arranged in a central plane and are embedded on all sides in an encapsulating material, e.g., ethylene vinyl acetate copolymer (EVA). The front side of such a solar module is formed, for example, by a glass pane as a front cover that covers the EVA embedment of the solar cells at the front side (top side) and provides good light transparency, weather resistance, scratch resistance in regard to environmental loads and cleaning actions and also a good electrical insulation of the solar cells toward the front side (top side) environment and a good mechanical stability. For covering the rear side of the solar modules, generally a rear-side film is employed which must not fulfill the specifications of the front side in regard to light transparency but must provide high specifications in regard to weather resistance, dielectric strength, and compatibility relative to the EVA embedment.
In particular, the rear-side film must be capable of being joined with the EVA embedment of the solar cells in the packet to a fixed and encapsulated unit by a lamination process in which the initially layered, loosely stacked materials at a temperature of 140° C. to 160° C. are joined with each other. In this context, the EVA is heated far beyond its softening temperatures so that it encloses the solar cells and binds to the front cover as well as to the rear-side film. Moreover, the EVA due to chemical additives or by radiation is crosslinked to a transparent material and to good weather resistance. The rear-side film is participating in this process as “outer shell”, with the requirement of a corresponding temperature resistance with exclusion of shrinkage processes for pre-stretched rear-side film.
In the context of high temperature resistance, rear-side films on the basis of polyester are conventionally used which however exhibit a bad compatibility relative to EVA and are to be provided with adhesive coatings. Also, the limited long-term hydrolysis resistance of this material with a tendency to water adsorption and brittleness is disadvantageous. A multi-layered configuration of such a rear-side film with intermediate and/or cover layers as well as relatively high shrinkage during module manufacture indicate significant disadvantages of the polyester material.
EP 2 390 093 B1 discloses in contrast thereto a rear-side film on the basis of polyethylene, wherein a material of polyethylene homopolymers and copolymers is joined directly, without adhesive layers, by co-extrusion. Polyethylene material has the benefit of a good compatibility with EVA within the solar module configuration. A problem is however the temperature resistance of the polyethylene material that is to be raised by select material components and in particular also by crosslinking of the material. Aside from the crosslinking expenditure, the limited temperature resistance not only during the EVA embedding process but also in the long run remains however critical when a solar module is to be used outdoors over decades, for example.
Object of the invention is to find a rear-side film for solar modules which can be produced to be weather-resistant and thermally robust as well as cost efficient and that enables in particular the production of resistant and cost-efficient solar modules with decade-long functionality.